There are many polyfunctional compounds, including diols and aromatic diamines, which are indicated to be useful as chain extenders in the preparation of polyurethane, polyurea, and polyurethane-urea polymers and/or as curing agents for epoxy resins. None of these compounds has a reactivity such as to make it universally ideal, and many fail to provide satisfactory properties in the products made by their use. Thus, there is still a need to find new compounds capable of serving as chain extenders or curing agents. U.S. Pat. No. 4,806,616 teaches the use of certain N,N′-dialkylphenylenediamines as chain extenders in preparing polyurethanes and polyureas. In this connection, also see for example U.S. Pat. No. 4,528,363, which teaches the use of secondary aliphatic diamines as part of a resin binder, and U.S. Pat. No. 6,218,480 B1, which discloses use of aromatic diamines as hardeners for polyurethanes. Secondary aromatic diamines have also been used as anti-degradants for rubber; see U.S. Pat. No. 4,900,868.
Imines are often formed from combination of a primary amine and an aldehyde or ketone. Such imines can be used as flavors (see U.S. Pat. No. 3,625,710) or fragrances (see EP 1067116).
To date, it has not been found possible to obtain aromatic diimines having groups with two or more carbon atoms from aromatic primary diamines where the two amino groups are either on one phenyl ring, or one amino group is on each of two phenyl rings, where the two phenyl rings are connected via an alkylene bridge, and in which each position ortho (immediately adjacent) to each amino group bears a hydrocarbyl substituent. Attempts to prepare diimines via reaction of such primary diamines with acetaldehyde or acetone in the presence or absence of catalysts have not worked; see U.S. Pat. Nos. 5,041,668 and 5,008,453. It had been indicated that the presence of an aryl group on the nitrogen or carbon of an imine group stabilized the imine; however, it has been reported that at least some of the compounds previously believed to be stable aromatic imines had been misidentified and were really polymers formed from unstable imines. In this connection, see Distefano et al., J. Chem. Soc. Perkin Trans. II, 1985, pp. 1623-1627.
It would be desirable to have routes to such diimines, and to have routes to aromatic secondary diamines that can be obtained from such diimines. There is a growing need for chain extenders with slower cure rates, so it would be a further advantage if these aromatic secondary diamines exhibited slower curing rates than those of presently available chain extenders.